Formation of chromium-containing layers on ferrous surfaces



Patented Oct. 22, 1940 UNITED STATES FORMATION OF CHROMIUM-CONTAININGOLAYERS ON FERROUS SURFACES Karl Daeves and Gottfried Becker, Dusseldorf,and Fritz Steinberg, Krefeld, Germany No Drawing. Application July 18,1938, Serial No. 219,920. In Germany July 21, 1937 4Claims.

' This invention relates to the formation of chromium-containing layerson surfaces of ferrous articles.

In the art of forming such layers, a recent development is the use ofgaseous chromium chloride (CrClz), which is caused to act on the ferroussurface at a high temperature.

The primary object of the present invention is to produce a more uniformand heavier layer m by means of gaseous chromium chloride.

Another object of the invention is to distribute the chromium chloridegas uniformly over the surface to be treated.

Yet another object is to render more appro- 5 priate the concentrationof the chromium chloride gas during its attack on the ferrous surface.

Gaseous chromium chloride is heavy, and therefore if the gas is allowedto flow continuously through a furnace containing the article 20 thefurnace must be suitably shaped or guiding surfaces be provided in orderto ensure that the chromium chloride'flows uniformly over the surface orsurfaces to be treated. If the article is packed with chromium chloridesalt in a closed 25 container and the whole is heated, it is diflicultto obtain satisfactory results. In the first place, the salt isextremely sensitive to moisture, and in the second place, it is used upas the process continues, so that, even if it is mixed with sand orequivalent refractory material, it ceases to afford an adequatesupporting bed for the heated article, which is liable to warp or becomedistorted when it is not properly supported.

According to the invention, the gaseous chro- 35 mium chloride is causedto act on the surface to be treated through a porous ceramic material,acting as a carrier, a transmitting medium, in-

stead of acting directly and freely on that sur face. In this way, moreuniform and heavier 40 layers containing chromium are produced, and theprocess is substantially facilitated. The carrier may advantageouslyconsist of porcelain or sillimanite, since these (particularlyporcelain) consist primarily of constituents which are not at- 45 tackedby chromium chloride, but also contain constituents which are soattacked. The action of chromium chloride on such a carrier leads to theproduction of very fine pores and passages by which flowing gas is splitup into a state of 50 very fine subdivision or in which chromiumchloride can be condensed and subsequently volatilised. Such fine poresor passages are, of course, additional to the pores formed by the spacesbetween the pieces in the carrier material. Nat- 5 urally it is alsopossible, and in some cases advantageous, to start with a material thepieces of which are already porous. A further and important advantageobtained by using porcelain or sillimanite is that they contain no, orvery few, constituents which form compounds 5 with chromium chloride,such for example as alkali or alkaline earth silicates. Ceramicmaterials which contain such constituents should be avoided, since theyrender much of the chromium inoperative and thus render the processuneconomical. Furthermore, in selecting a suitable material, one that isitself consumed during the treatment should be avoided, as otherwise thecarrier will not always form a good support for the articles packed init.

The size of the pieces forming the carrier material may vary with thedimensions of the article to be treated. The carrier material should inany case be granular and not sandy or powdered, and the pieces should beof such a size that the parts to be treated are as far as possibleuniformly surrounded by the carrier material. For articles of moderatedimensions carrier material composed of pieces of from 3 to 6 mm. havebeen found particularly suitable.

The invention may be carried into effect in various ways. One waycomprises charging the carrier material with chromium chloride andpacking the article with the charged carrier material in a container,which is then closed and heated. Instead of using chromium chlorideitself, substances capable of yielding chromium chloride may be mixedwith the carrier or charged into the same container, and the chromiumchloride may then be formed in situ, Thus chromium chloride salt may beplaced in the container and hydrogen may be led through the containerduring the heating, so that chromium chloride gas is formed. I

Again, the article to be treated may be placed in a furnace andsurrounded by the carrier materiaLand chromium chloride gas may bepassed through the furnace. The gas flows through the carrier to reachthe ferrous surface, and produces a particularly uniform and strongsurface 5 layer. It seems that the chromium chloride concentration isrendered more satisfactory by the use of the carrier material.

The chromium chloride gas may be produced in any suitable way; forexample, chromium chloride salt may be heated in the presence ofhydrogen as described above, or gaseous hydrogen chloride may be passedover heated chromium or alloys or compounds of chromium.

The duration of the treatment depends on 5 the thickness ofthe layerdesired and on the reaction temperature. Thus treatment for four hoursat from 960 to 980 C. may hen to produce the same thickness of layer asis produced in from one to two hours at from 1000 to 1080' O.

In order to prevent any oxidation of the articles, it is desirable tocarrier: the treatment said surface with heated gaseous chromiumcliioride through said carrier.

3. In the formation of a chromium-containing layer on the surface of aferrous article, the steps which comprise charging with chromiumchloride a ceramic carrier material selected from the group of porcelainand sillimanite, packing said article, with said charged carriermaterial surrounding and in contact therewith, in a container, closingsaid container, and heatin: said clam container to cause the chargedchromium chloride to volatilize and act on said article through saidcarrier material.

4. Inxthe formation of a chromium-containing 1o layer on the surface ofa ferrous article, the steps which comprise introducing said articleinto a container, packing said article in said container with a ceramiccarriermateriai selected from the group consisting of porcelain andsiilimanite, and 15 at least one substance capable of yielding chromiumchloride gas, sin-rounding and in contact with said article, andproducing chromium chloride gas in said container from said substance,

to cause said gas to act on said article through 20 said carriermaterial.

xam. canvas 7 GO'I'I'FRIED BECKER. ram

